Download Finding the epicenter of an earthquake Background Information

Finding the epicenter of an earthquake
Background Information: Movement within the earth along a fault or between plates is the cause of
shock waves to spread out in all directions. The point at which the movement occurs is called the focus.
Above the focus, on the surface of the earth is the epicenter. There are two distinct types of shock
waves that are used to determine an earthquake’s epicenter: P-waves (primary waves) move in a push
and pull fashion, travel the fastest and arrive at seismic stations first. S-waves (shear waves or
secondary waves) move in a side-to-side fashion, travel slower than P-waves and arrive at a seismic
station after the P-waves. Each type of wave is affected by the density of the earth material in which it
travels. P-waves can travel through both the solid crust and mantle and the molten outer core. S-waves
can also travel through the solid parts of the earth, but not through liquid or molten material. Seismic
stations throughout the world can detect distant earthquake waves. The key is to determine, from the
seismograph reading, the time lapse between the arrival of the first P-wave and the first S-wave. A
seismologist can use data from multiple seismic stations to determine an earthquake’s epicenter. Data
from three stations are necessary to do this. When seismograph data from stations at different
locations is compared, the precise location of an earthquake’s epicenter can be determined.
Materials:

Drawing compass with pencil

Graph that plots distance to epicenter (X-axis) and Distance in arrival time between P and S
waves

United States map
Procedure:
1. Examine the three seismograms and record the lapse time in minutes between the
arrival of the first P-wave and the arrival of the first S-wave. Do this for each seismic
station.
2. Use the Travel-Time graph to determine the distance in miles each station is from the
epicenter of the earthquake. To do this, find the time in minutes (Y-axis) and go
horizontally across to the curve. Now go vertically downward to the X-axis and record
the corresponding distance in miles.
3. Use the distance from the epicenter in miles for the first seismic station. Place the
compass point exactly on the “0” mark of the scale. Widen the compass so the pencil
point matches the distance.
4. Maintaining the compass width, place the point of the compass on the map at the
location of the first seismic station. Sketch a complete circle.
5. Repeat steps 3 and 4 for the remaining two seismic stations.
6. The three circles should intersect at a point. This is the location of the earthquake’s
epicenter.
Seismograph
Station
Oklahoma City
Denver
Tampa
Chicago
Arrival (clock time)
“P” wave
“S” wave
Difference
in arrival
time
min & sec
Distance
to
epicenter